1. Field of the Invention
The present invention relates to a DTMF (Dual Tone Multiple Frequency) receiver.
2. Description of the Prior Art
A DTMF signal is a signal used to transmit a telephone number, etc. from a push-button telephone or the like to a telephone exchange. Recently, a DTMF signal has also come to be used for transmission and reception of information after establishing a connection between telephones. Therefore, some facsimile machines and multifunctional telephones for connection to a public switched telephone network are equipped with a DTMF receiver.
In a DTMF signal, one of four lower frequency tones is combined with one of four higher frequency tones. The following four frequencies f.sub.A to f.sub.D are allocated to the tones of the lower frequency group.
f.sub.A =697 Hz PA1 f.sub.B =770 Hz PA1 f.sub.C =852 Hz PA1 f.sub.D =941 Hz PA1 f.sub.E =1209 Hz PA1 f.sub.F =1336 Hz PA1 f.sub.G =1477 Hz PA1 f.sub.H =1633 Hz
The following four frequencies f.sub.E to f.sub.H are allocated to the tones of the higher frequency group.
For example, the DTMF signal representing the dial number "1" is composed of a combination of tone frequencies of 697 Hz and 1209 Hz.
FIG. 5 shows a conventional DTMF receiver. The DTMF receiver of FIG. 5 includes eight band-pass filters 110 to 117 which are connected to an input terminal 100. Each band-pass filter corresponds to one of the above-mentioned eight tone frequencies f.sub.A to f.sub.H. In each band-pass filter, the center frequency of the pass band is set so as to be equal to the corresponding tone frequency. Connected to the band-pass filters are multipliers 120 to 127 by which the strengths of the output signals of the band-pass filters 110 to 117 are squared, respectively. The outputs of the multipliers 120 to 127 are supplied to level detectors 140 to 147 through smoothing low-pass filters 130 to 137, respectively. The output signals from the level detectors 140 to 147 are input to a coder 150.
The operation of the DTMF receiver of FIG. 5 will be described, taking as an example a case in which the DTMF signal representing the dial number "1" is supplied to the input terminal 100. The tone frequencies of 697 Hz and 1209 Hz which constitute the DTMF signal representing the dial number "1" pass through the band-pass filters 110 and 114, respectively. Therefore, the outputs of the level detectors 140 and 144 becomes HIGH, while the outputs of the other level detectors remain LOW. On the basis of the output signals of the level detectors 140 to 147, the coder 150 outputs a code indicating that the DTMF signal representing the dial number "1" has been entered.
As seen from the above description, a conventional DTMF receiver requires a large number of components including eight band-pass filters and eight smoothing low-pass filters. Also, since the tone frequencies are close to each other both in the low frequency group and in the high frequency group, the band-pass filters are required to have sharp and precise frequency-amplitude characteristics. Thus, a conventional DTMF receiver is very complex in construction.